Printfn值不是函数，不能在F＃的交互窗口中应用

Question

This is probably something simple, but I have a .fs file with a simple sample function.

let printHello = printfn "%A" "Hello"

I have set the search path to the .fs file using

>#I "PathToMyFSFile"

I have loaded my file using

>#load "Test.fs"

Which worked fine. Now I want to call the function which actually prints the hello to screen, but thats turning out to be too difficult

> Test.printHello;;
val it : unit = ()

Tried Test.printHello();; as well but doesn't work. How do I actually make it print "Hello" to screen?

Answer 1

your current printHello isn't actually a function. To make it a function you need to do

let printHello() = printfn "%A" "Hello"

noice the () . Then everything should work.

EDIT:

When the compiler sees your definition

let printHello = printfn "%A" "Hello"

it passes it as a simple data term. For example, consider this program:

let printHello = printfn "%A" "Hello"
printfn "World"
printHello

This will print "Hello" then "World" . printHello just has unit type, so does nothing. Compare it to

let printHello() = printfn "%A" "Hello"
printfn "World"
printHello()

Here printHello is a function. In this case, the function is only executed when it is explicitly called, so this prints "World" then "Hello" .

Answer 2

As John already said, your printHello isn't a function - it is a value of type unit . When you give printfn all the required arguments (as you did), it does the imperative operation and return unit (which is a type with only a single value written as () ). You can see that writing that declaration does the printing immediately:

> let printHello = printfn "%A" "Hello";;
"Hello"
val printHello : unit = ()

When you use printHello later, it simply refers to this unit value (which does not carry any information).

If you want to make it a function (of type unit -> unit ) that will do something each time it is executed, then you can use the sample that John posted.

The function printfn was not partially applied , because you gave it all the parameters it required (so it could just print immediately). If you wanted to use partial application, you could use something like:

> let printHello = printfn "%s %s" "Hello";; // Note - didn't give value for second %s
val printHello : string -> unit

Now printHello is a function that waits for the second parameter and then runs:

> printHello "World";;
Hello World
val it : unit = ()

> printHello "F#";;
Hello F#
val it : unit = ()

Answer 3

As has already been said in other answers, "printHello" set to () is unit, the return value of printfn is () , console print is side-effect.

use Lazy:

let printHello = Lazy (fun () ->printfn "%A" "Hello")

DEMO

> Test.printHello.Value;;
"Hello"
val it : unit = ()
> Test.printHello.Value;;
val it : unit = () //only once

use Seq:

let printHello = seq { printfn "%A" "Hello"; yield ()}

DEMO

> Test.printHello;;
"Hello"
val it : seq<unit> = seq [null]
> Test.printHello;;
"Hello"
val it : seq<unit> = seq [null] //many times